1. Field of the Invention
The present invention relates to residues which are obtained in the purification of flue gases from incineration plants, in particular from fly ash, which contain heavy metals and which therefore cannot be disposed of directly in landfills.
2. Description of the Background
DE-B 33 20 466 (U.S. Pat. No. 4,617,180) discloses the extraction of flue dusts containing heavy metals with an acidic solution and then effecting precipitation of the heavy metals with a precipitating agent. The resulting precipitate having a high heavy metal content is fed to a metal recovery stage. The extracted residue, which still contains small amounts of heavy metals, is recycled to the incineration plant and incorporated in the slag of the plant under the action of heat. This method of disposal is expensive and may lead to an accumulation of heavy metals in the incineration plant.
EP-A 0 482 335 discloses that the heavy metals remaining in the extraction residue after the acidic extraction can be immobilized with an aqueous solution of a precipitating agent. If the pulverulent extraction residue rendered inert in this manner is conveyed to a landfill, it may be transported, in the dry state, by the wind and by the water when it rains. Here too, the heavy metals are precipitated from the extract.
In both known processes, it is necessary to find an outlet for the precipitate having a high heavy metal content. If, for economic or technical reasons, recovery is impossible, this precipitate must be disposed of in an environmentally compatible manner, in accordance with particular local regulations. A need therefore continues to exist for an acceptable method of disposing of precipitated heavy metals obtained from fly ash.